Rockzmom, Bitpicker, of course, you are right when you are speaking about the conditions of learning. I suppose, if detach a person from the conditions, then a success of learning of languages depends on two ingredients.
The first is your aptitude (your memory, memory, memory and a bit of logical thinking). It is 20% of the success.
The second is your willpower. It is 20%.
Of course, it is not all. The rest are the conditions.
The third is a talented teacher. (this person have to find the best way to your mind. He/She have to choose the right method of teaching for you. This ability is a talent. Not all teachers have one! I know a Russian person who knows English very well, but she cannot explain, and don’t like do the explanation). Instead of or in addition to the teacher are text-books. It is 30% .
The fourth is practice. It is 30%. (The living in foreign country it is the best way of practice )
I know that the numerical values are very roughly. Just, when we learn own languages we have the best ways of all these components. But anyway, every of us have memory and a bit of logical thinking. So every of us have the aptitude. But the another components nobody can cancel